Dinoflagellate cysts can be smooth or bear granules, ridges, indentations, raised crests or develop short spines, processes or horns (Fig. 10.4). Processes can be plate centred or form groups. Tabular ornament is sutural if it defines plate boundaries or intertabular if it defines the central parts of plates. Processes that are situated at the intersection of paraplate boundaries are gonal and those along boundaries are intergonal.
Where a reflected cingulum is present, that portion apical to it is called the epicyst and the antapical portion is called the hypocyst (Fig. 10.3b).
The function of the cyst is demonstrated by the presence of an escape hole, called an archaeopyle. This is formed by the removal of one or several plates (thereby comprising an operculum) normally from the apical series, the precingular series, an anterior intercalary plate or a combination of these. The form and position of the archaeopyle is constant within a genus.
Further investigation is needed into the functional and ecological significance of cyst morphology. The processes of chorate cysts and the pericoels of cavate forms may both be mechanisms to minimize the downward sinking of oceanic species. If the cysts of such forms were to sink far below the photic zone before excystment, their chances of survival would be reduced. In laboratory cultures, similar morphotypes of cyst are known to produce markedly different motile cells and apparently identical motile cells can produce very different cysts. In Recent sediments cyst morphology may be directly related to salinity. Tectatodinium and Spiniferites (Figs 10.3e, 10.9b) are round in outline in normal marine conditions yet cruciform in low-salinity conditions. Operculodinium (Fig. 10.7c) species are known to have reduced numbers of processes in low-salinity environments. For these reasons, in fossil assemblages, it is unlikely that there is a simple relationship between the cysts preserved and the motile cells originally present.
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